Coating



Patented Dec. 12, 1933 UNITED STATES COATING Martin Tosterud, Arnold, Pa., asslgnor to Aluminum Company of America, Pittsburgh, Pa a corporation of Pennsylvania No Drawing. Application May 2e, 19::

Serial No. 013,792

3 Claims.

The invention relates to the production of chemical coatings on aluminum and aluminum base alloys, the latter being herein and in the appended claims comprehended by the term aluminum".

The characteristic color of aluminum, although pleasing in many instances, is not always suitable to the purposes to which that metal is put. Consequently it is necessary to provide coatings on the aluminum which are integral with the metal, which are readily applied and which can be produced at relatively low cost. Likewise, as where the aluminum is subjected to outdoor conditions, it is often necessary and desirable to provide thereon coatings which are permanent, which are color-fast and which aflord to the aluminum 9. degree of protection against the elements. It is to these and similar ends that the present invention is directed; it having the particular object of chemically producing on aluminum a grey coating.

In accordance with the invention, grey coatings are chemically produced upon aluminum surfaces by treating the surface with a solution containing an alkali metal stannate and an alkali metal dichromate. The depth of color produced depends somewhat upon the strength of solution employed, stronger solutions producing progressively lighter coatings when other conditions of treatment are equal.

In practicing the invention, the aluminum surface to be treated is preferably first cleaned of grease and other extraneous matter. This may be readily accomplished by the use of solvents, such as gasoline, or by mechanical abrasion when a very smooth surface is not required. The aluminum article is then immersed in the solution and left there for a time which may be predetermined to meet particular conditions but which is usually on the order of about 1 to 15 minutes. After the coating is completely formed, the aluminum article is removed from the solution, washed to remove traces thereof, and dried. The article is then ready for use without further treatment. The result is a smooth surface, light to dark grey in color, and oflering resistance to the action of mildly corrosive agents. v i

The solution in which the aluminum surface is treated is one containing an alkali metal stannate and an alkali metal dichromate or the equivalent alkali metal chromate. Sodium stannate and potassium dichromate is the combination preferred, but the other soluble alkali metal stannates and chromates or dichromates give excellent results. In the solution, the alkali metal stannate may be present in amounts of about 0.1 to 15 per cent by weight; the alkali metal dichromates or chromates in amount of about 0.1 to 6 per cent by weight. In the speciflcation and in the appended claims the term alkali metal dichromate" defines not only the alkali metal dichromates but likewise the alkali metal chromates.

The solutions, in order to be effective when used in the rapid treatment of large numbers of aluminum articles, should be held at temperatures between about 90 to 100 centigrade since at these temperatures the coating is formed rapidly and evenly and tends to adhere with greater tenacity to the aluminum surface on which it builds. Treatments in cold solutions are satisfactory but the time at treatment is considerably extended.

As a specific example of the method in which the invention may be practiced, a good hard coating of a light grey color has been placed upon aluminum alloy surfaces by first cleaning the aluminum surface to remove therefrom extraneous matter, and then immersing the aluminum surface in a solution containig 2 per cent by weight of sodium stannate andl per cent by weight of potassium dichromate. The solution was held at a temperature of about 9'! centi- "grade and the specimen was immersed for a period of about 15 minutes. The aluminum, having been taken from the solution, was washed in water to remove excess solution from the coating surface and was thereafter dried.

The methods of the invention are particularly eflective in the treatment of large aluminum surfaces, such as the surfaces of aluminum'shingles, spandrels and similar objects; the ease of treatment and its low cost enabling the production of relatively inexpensive aluminum articles.

I claim:

1. A method of chemically coating aluminum by treating the aluminum surface with a solution of an alkali metal stannate and an alkali metal dichromate.

2. A method of chemically coating aluminum by treating the aluminum surface with a solution containing 0.1 to 15 per cent by weight of alkali 1 metal stannate and 0.1 to 6 per cent by weight of alkali metal dichromate.

3. A method of chemically coating aluminum by treating the aluminum surface with a solution containing 0.1 to 15 per cent by weight of alkali metal stannate and 0.1 to 6.0 per cent by weight of alkali metal dichromate, the temperature of the solution being about 90 to 100' centigrade.

. MARTIN TOBTERUD. 

